1. Field
The present disclosure generally relates to systems and methods of reducing the exhaust noise from a jet engine and, in particular, reducing the noise using high frequency actuation of the exhaust nozzle vanes.
2. Description of the Related Art
High-performance fighter aircraft are powered with low-bypass-ratio turbofan or “jet” engines such as shown in FIG. 1 and variable-geometry nozzles as seen in FIG. 2. The jet flows which emanate from these nozzles are supersonic (at high power settings) and, at most conditions, over-expanded (i.e. the static pressure at the nozzle exit is lower than ambient static pressure).
The noise emitted by high performance jet engines is due to the turbulent mixing of air in the supersonic exhaust jet, as well as unsteady motion of shock waves, and shock wave-turbulence interaction. Reduction of jet noise in the past has been achieved via enhancement of mixing in the jet shear layer through the use of chevrons, tabs, fluidic injection, etc. This increases the noise in the immediate near field, but also increases the frequency of the radiated noise. Since the atmosphere absorbs high frequency sound much more rapidly than low frequencies, the mixing enhancement results in lower noise in the far field. However, chevrons, tabs, etc. generally cause additional drag on the aircraft and increase the weight and complexity of the nozzle. Thus, these devices are not generally employed on high-performance military aircraft.